A radio electric signal modulated according to the orthogonal frequency division multiplexing OFDM is distributed into a great number of subcarriers in a wide frequency band compared to the separation between the subcarriers. The signal is thus emitted by an emitter on different subcarriers such that the signal received by a receiver can be processed in order to restore the emitted signal in spite of potential destructive interferences due to the multiple propagation paths of the signal.
Noise and interferences undergone by the signal during its propagation between the emitter and the receiver degrade the signal reception. An insufficient treatment of noise and interferences leads to a high decoding error rate.
In the state of the art, noise and interferences can be processed by comparing a signal resulting from the combination of signals respectively received by the antennas with an estimate of the emitted signal.
In FIG. 1, an example is given in the case of two receive antennas having received signals r1 and r2 respectively. Weightings w1 and w2 are applied to signals r1 and r2 respectively, and then the two weighting signals are summed in order to obtain a signal r′ from which an estimate d of the emitted signal is subtracted. A function of the resulting difference signal ε is minimized. For example, the sum of the squared standard of the values of ε for the symbols of the frame is minimized. Finally, the signal r′ is processed in order to estimate the emitted signal.
The estimate of the emitted signal can be known a priori from the receiver, as is the case for example for pilot symbols inserted into each emitted frame and known from the emitter and receiver. The estimate of the emitted signal can also be obtained by a preliminary processing of signals received on the different antennas, for example using a technique known as MRC (Maximum Ratio Combining).
The processing described in FIG. 1 can be applied iteratively, with the data estimated at iteration i being partly or entirely used as an estimate d of the emitted signal for the following iteration i+1. Noise and interferences are reduced by estimating the emitted signal at each iteration and by calculating the difference, for each symbol of the OFDM frame, between the estimate of the emitted signal and the weighted sum of signals respectively received by antennas until this difference tends towards a limit value.
The combination of signals received by the different antennas helps suppress or reduce the level of interferences remaining in the resulting signal. By comparing this weighted signal with the emitted signal (more precisely the estimate of the emitted signal), weighting to be applied to each of the signals received by the different antennas can be optimized such as to suppress or reduce existing interferences.